The present invention relates to a papermaking fabric.
In the conventional papermaking process carried out by using a fourdrinier wire screen, a slurry of raw material is supplied onto an endless papermaking fabric or cloth running continuously. The papermaking cloth separates cellulosic fibers from the slurry to thereby form a moist paper web thereon. From this viewpoint, it may be regarded that the papermaking cloth functions as a filter for forming the moist paper web. Most apertures of the cloth also referred to as the drain orifices serve for separating water from the slurry. Further, in the case of a fourdrinier machine, the papermaking cloth serves also as a driving belt and is thus subjected to a tension exerted by the machine. In view of this, it is required that the papermaking cloth should exhibit an enhanced stability.
Among the several requirements imposed in connection with the papermaking cloth or fabric, the following are to be noted among others. Namely, the papermaking cloth is required to exhibit a high retention capability of retaining paper material with a minimum flow loss, no generation of wire marks, a high drainage capability with a reduced water retaining capacity, a high capability of abrasion resistance and an enhanced runability.
With a view to satisfying the above requirements imposed on the papermaking cloth, there have heretofore been made a variety of proposals. However, at the present state of the art, there is yet unavailable the papermaking cloth which satisfies the abovementioned requirements to the satisfactory extent.
By way of example, the finely woven papermaking cloth woven made using thin yarns in an attempt to enhance the raw material retention capability while preventing formation of the wire marks suffers from such shortcomings that the runability and the abrasion resistance capability are poor. In recent years, attempts have been tried to form the cloth of the papermaking surface from the wefts for thereby improving the raw material retention capability. The papermaking surface formed from the wefts is certainly advantageous in that the cloth or fabric is improved in respect to the drainage property because of little or no possibility of the drain apertures existing between the warps being directly blocked by the paper material. It is however noted that the wire marks become more noticeable because the inter-weft gaps are increased correspondingly.
As an approach to solve the above problem, it has been proposed to increase the number of the wefts forming the papermaking surface by disposing so-called floating yarns which are not woven into the texture of the fabric in the form of interweave with the warps and the wefts. This proposal is certainly an interesting technical idea from the standpoint of increasing the number of the wefts constituting the papermaking surface of the cloth, which idea can not however be applied to practical papermaking process, because the wefts not woven into the texture, i.e. the floating yarns tend to be displaced and collected together under a hydraulic pressure applied thereto upon charging of the slurry on the papermaking cloth. The result is that the papermaking surface can not be maintained flat or uniform.
The problem of the wire marks is more noticeable in the case of the single-layer cloth in which the wefts form projections on the papermaking surface.
There has also been proposed the use of a cloth of a combination weave (multi-layer fabric) in an effort to obtain a high drainage property and a papermaking surface of a fine mesh while ensuring a high abrasion resistance capability.
Recently, there also has arisen a tendency that the papermaking process is carried out with a higher speed with a view to increasing the efficiency of the papermaking process, which however presents additional new problems. The multi-layer fabric which can certainly exhibit advantageous effects unattainable with the single-layer fabric has a high water containing property which is primarily ascribable to the multi-layer structure. By way of example, when the endless screen formed of the multi-layer fabric is driven at a high speed, there will take place such a phenomenon that water droplets are caused to spill out particularly at the rotating turn-back rolls under a centrifugal force.
In this conjunction, it is noted that the single-layer fabric is essentially insusceptible to the phenomenon mentioned above due to the inherently small water containing capacity. However, the single-layer fabric is constituted by the wefts thereby, forming juxtaposed long crimps projecting on the running surface for protecting the warps from the abrasion. In other words, the papermaking surface is formed mainly by the long crimps of warps disposed in parallel in cooperation with only a small proportion of the weft knuckles.
Since the parallel rows of the long warp crimps extending in the machine direction coincide with the direction in which the fibers contained in the flow of the raw paper material ejected from an inlet pore are oriented, the fibers tend to be deposited within the longitudinal grooves formed between the long knuckles of the warps at an early stage of the papermaking process to thereby block the drain apertures or meshes, making difficult the separation and removal of water in the subsequent papermaking process.
To avoid the abovementioned problems, such measures as slow down of papermaking speed or an enforced vacuum dehydration must be performed, which in turn will result in a rapid abrasion of the papermaking fabric and bring about various difficulties such as described hereinafter.
After studies conducted by the inventor of the present application for solving the problems associated with the requirements for a high paper material retention capability, suppression of generation of the wire marks, a high drainage capability and a low water containing capacity, a high abrasion resistance capability and an improved runability and others, it has been found that these problems cannot be solved without increasing the density of wefts on papermaking surface of the fabric for the propose of improving retention capability, that a single-layer fabric should be used instead of a multi-layer fabric which has limitations for lowering water containing capability, and that these problems can not be solved merely by changing the weave pattern of the single-layer fabric from plain weave to twill weave, satin weave or others.
Referring to FIGS. 1A and 1B of the accompanying drawings which shows a texture of 3/1 satin woven fabric which is a typical one of the single-layer broken-twill woven fabrics known heretofore, the warp 1 extending from the bottom side of the wept A to the top side of the weft B intersects a warp 2 extending from the top side of the weft A to the bottom side of the weft B at a location between the wefts A and B. In FIGS. 1A and 1B the warps (Machine Direction (MD) yarns) are designated by alphabet symbols, namely, A, B, C and D, and the wefts (Cross Machine Direction (CMD) yarns) are designated by Arabic numerals, namely 1, 2, 3 and 4. FIG. 1A shows the unity textile design charts. A symbol "X" indicates the position where the warp is located on the weft as well as the position where the weft is woven into the texture of the warp, whereas a blank box having no mark "X" indicates the position where the weft is located on the warp. FIG. 1B shows the paperside plan view of the texture of 3/1 satin woven fabric.
Similar intersection of the warps 3 and 4 takes place between wefts C and D.
Due to the intersection of the two warps as described above, the gap between the wefts A and B as well as between the wefts C and D tend to be widened, while the gap between the wefts of B and C as well as between the wefts D and A tends to be narrowed because of absence of the intersection of the warps between these wefts.
Under the circumstances, difficulty is encountered in realizing the uniform mesh, involving non-uniform formation of the inter-weft gaps.
Additionally, it is noted that the warps 1 and 2 obliquely intersect each other between the wefts A and B to form a recess the surface of the woven fabric. The reason why such a recess is formed is, as shown in FIG. 1B, the weft A is pushed or urged toward the weft, and the weft B is pushed or urged toward the weft C, due to the force created by the intersecting warps 1 and 2. Such recess makes appearance between the wefts C and D as well. The presence of these recesses between the wefts in addition to the wide inter-weft gaps allows the fibers of the paper material to pass through the papermaking fabric to be lost uselessly (degradation in the paper material retention capability). Further, the fibers are likely to stick to the fabric (giving rise to sheet release layer from the fabric and deterioration in the surface quality of paper) and generate roughness in the paper surface (deterioration in smoothness and formation of the wire marks).
Parenthetically, the ordinary twill-woven fabric such as, for example a 2/1, 3/1 or 4/1 weave design is generally insusceptible to the occurrence of the non-uniformity in the distribution of the wefts mentioned hereinbefore. However, the wire marks tend to be noticeable in the oblique direction because of presence of the recesses in the oblique direction (twilling direction) and thus the twill-woven fabric is not suited for use as the papermaking sheet.
Besides, multi-layer twill-woven fabric exhibits a high water containing capacity because of presence of large voids, involving the problem of adhesion of water onto the paper surface and shower effect, difficulty encountered in repairing the papermaking fabric, inefficiency of the paper manufacture, heaviness and others. For these reasons, the multi-layer twill-woven fabric does not show satisfactory results.